1. Field of the Invention
The field of the invention is that of installations for the cleansing and filtering of liquids, notably water, of the type including a loop for the circulation of the water to be treated, including at least one tangential filtration membrane.
The installation of the invention can be applied preferably to the cleansing of surface water but those skilled in the art could contemplate using it for the treatment of waste or untreated water at other steps of the cleansing chain or, again, for the treatment of other liquids.
2. Description of the Prior Art
Water treatment with a view to the distribution of water for consumption, taking account of currently prevailing standards, has the following main aims:
the removal of the suspended matter; PA1 the removal of organic matter; PA1 the removal of harmful ions; PA1 sterilization.
The standard treatment chain mostly includes a succession of physical/chemical steps of the coagulation-flocculation-decantation-filtration type.
The filtration step, for which the present invention proposes a new implementing installation, makes it possible to refine the treatment and to retain most of the residual reagents.
In a known way, the term tangential filtration shall be applied to any filtration where the fluid to be filtered circulates under pressure in parallel to the filtering surface as opposed to front filtration where the fluid flows perpendicularly to this very same surface.
Tangential filtration with mineral membranes has many advantages over standard type filtration. In particular:
the quality of chemical, thermal and bacteriological inertia of the mineral membranes;
the limiting of excessively fast clogging through the self-cleaning of the filtering surface by the fact that the non-filtered particles are entrained or carried along owing to the tangential circulation speed of the untreated water.
Tangential ultrafiltration and microfiltration on mineral membranes are considered to be excellent methods of treating water and making it potable for small installations. These methods are generally implemented in a circulation loop of the liquid to be treated, a part of the liquid (the permeate) going through the membrane when the filtrate remains in the loop.
Ultrafiltration is a method of separation, under pressure, of the dissolved or suspended macromolecules, bacteria (with a diameter of 0.5 to 10 microns), viruses (minimum diameter: 20 nanometers) and other microorganisms, by an aysmmetrical membrane in which the size of the pores varies between 1 nanometer and 0.1 microns.
For microfiltration, the diameter of the pores varies between 0.1 and 10 microns.
One of the crucial problems encountered in tangential membranes is clogging, which may take many forms:
surface clogging: when the filtration takes place, there is always an immobile boundary layer of solution, in contact with the membrane, from which water is continually extracted and in which the solute retained collects to abnormally high levels of concentration. This phenomenon is called polarization of concentration and leads to the formation of a so-called polarization layer. The speed of circulation of the fluid that flows tangentially to the membrane should be high enough to prompt a self-cleaning of the membrane and restrict the formation of the polarization layer and, hence, the gradual clogging of the membrane by precipitation of the products on the surface of this membrane;
internal clogging: among the particles capable of going through a microfiltration membrane there are, in particular, colloids with a size that is very slightly smaller than that of the pores of the membrane into which they penetrate and flocculate, in prompting an irreversible clogging:
chemical clogging: caused essentially by hydrophobic substances such as proteins and oily particles.
Following the filtering operation, it is known that the water should then be rid of pathogenic microorganisms by means of oxidizing agents (ozone, chlorine etc.) or UV rays or again refining treatment with granular or powdered active carbon (for the removal of micropolluting agents, heavy metals as trace elements, and unwholesome odors and flavors).
The use of ozone is recognized as being efficient not only for bactericidal and virus-killing applications, but also in combined steps of ozonization/coagulation, ozonization/floatation (ozoflottation) and ozonization/adsorption on filtering media, not to mention other more standard applications such as iron extraction and demanganization or the removal of flavors and odors. Finally, it is known that ozone has an oxidizing effect on a number of micropolluting agents (such as phenols, certain detergents, etc.) (see B. Langlais, "Nouveau developpement de l'ozonation en eau potable et technologie approprie" (New Development of Ozonization in Potable Water and Appropriate Technology) in L'Eau, l'industrie, les nuisances, No. 109, April 1987, pp. 28 to 30).
However, ozonization has two major limitations:
when an oxidizing agent is used, chlorine is generally chosen to play a role of disinfection (instead of ozone, by UV radiation etc.) because of its remanent effect which circumvents possible recontamination during the storage of the treated water or during its transfer into the distribution conduits;
furthermore, the use of a gas is highly unadvisable in tangential membrane filtration loops. Indeed, the gas is injected in the form of bubbles so as to achieve a maximum gas/liquid transfer. Now, it is known that a clogging of the membranes may occur by the expansion of the bubbles in the membrane. As described by F. Duclert and M. Rumeau in "Microfiltration d'eau sur membranes minerales. Influence des sels et des gaz" (Microfiltration of Water in Mineral Membranes. Influence of Salts and Gases), Liquides Magazine, No. 9, January-February 1989 concerning air dissolved in water, in passing through the membrane, the pressurized solution is released by atmospheric pressure and the dissolved air "precipitates" by a phenomenon of effervescence, giving rise to microbubbles of air within the pores. By coalescence, these bubbles may then prompt an occlusion of the pores of the membrane.